1. Field of the Invention
The present invention relates to an image forming apparatus which forms a multi-color image by transferring toner images of different colors formed on at least one image carrier onto an endless transfer belt while superimposing thereon and then transferring a superimposed color toner image from the transfer belt onto a recording medium, or by transferring toner images of different colors from at least one image carrier onto a recording medium carried and conveyed on an endless transfer belt. The present invention also relates to a position shift detection and correction method for detecting and correcting a position shift in a color toner image formed on a transfer belt or a recording medium carried and conveyed on the transfer belt caused by an uneven thickness of the transfer belt.
2. Discussion of the Related Art
In an image forming apparatus, such as, a copying machine, a printer, a facsimile machine, a multifunctional image forming apparatus, or other similar image forming apparatuses, a multi-color image is generally formed on a recording medium, such as, a transfer sheet, by the following process: forming toner images of different colors on at least one image carrier; transferring the toner images from the at least one image carrier onto an endless transfer belt while superimposing thereon and then transferring a superimposed color toner image from the transfer belt onto a recording medium, or transferring the toner images of different colors from the at least one image carrier onto a recording medium carried and conveyed on a transfer belt; and fixing the color toner image on the recording medium. In such an image forming apparatus, positions of transferred toner images are shifted on a transfer belt or a recording medium carried and conveyed on a transfer belt due to a speed variation of the transfer belt. As a result, a color shift (color misregistration) occurs in a color toner image, thereby deteriorating image quality.
In order to solve the above-described color shift problem, a speed variation of a transfer belt is detected while measuring a surface velocity of the transfer belt. The rotational speed of a motor, which drives a drive roller that drives the transfer belt to rotate, is controlled in real time based on the detection result of the speed variation of the transfer belt such that the surface velocity of the transfer belt remains constant.
Generally, the speed variation of the transfer belt is caused by an uneven thickness of the transfer belt in its circumferential direction, a speed variation of an image carrier that rotates while contacting the transfer belt, a speed variation of a drive roller that drives the transfer belt to rotate, and the like. Thus, when the speed variation of the transfer belt is detected by measuring the surface velocity of the transfer belt, the speed variation of the transfer belt includes an uneven thickness component of the transfer belt, a speed variation component of the image carrier, and a speed variation component of the drive roller. Therefore, the speed variation of the transfer belt changes every rotation cycle. In this condition, it is required that the speed variation of the transfer belt be detected every time an image forming operation is performed and that the rotational speed of the drive roller for driving the transfer belt be controlled based on the detection result. Such speed control of the drive roller requires very precise control and parts manufactured to a high degree of accuracy, which increases a cost of using the speed control.
If only a speed variation component of a transfer belt caused by an uneven thickness of the transfer belt can be determined and detected while eliminating, for example, a speed variation component of an image carrier and a speed variation component of a drive roller, the rotational speed of the drive roller can be set such that the speed of the transfer belt becomes constant based on the detection result of one time detection operation for the speed variation of the transfer belt. In this case, a speed control of the drive roller can be performed in a simple manner. However, such control is not known in the art.
Published Japanese patent application No. 10-186787 proposes a technique in which only a speed variation component caused by an uneven thickness of a transfer belt is determined by performing a low-pass filter processing. However, a specific construction and method are not described.
Therefore, it is desirable to provide an image forming apparatus that detects and corrects a position shift in a color toner image formed on a transfer belt or a recording medium carried and conveyed on the transfer belt according to an uneven thickness of the transfer belt.
It is further desirable to provide a position shift detection and correction method for detecting and correcting a position shift in a color toner image formed on a transfer belt or a recording medium carried and conveyed on the transfer belt according to an uneven thickness of the transfer belt.